The present invention relates to spindle-shaped magnetic iron-based alloy particles containing cobalt and iron as the main ingredients, spindle-shaped goethite particles containing cobalt, and a process for producing the same. Particularly, the present invention relates to spindle-shaped fine magnetic iron-based alloy particles containing cobalt and iron as the main ingredient which have a uniform particle size, especially the size distribution (standard deviation/major axis diameter) of not more than 0.25, no inclusion of dendrites, a high coercive force, especially a coercive force (Hc) of 1720 to 2500 Oe and an excellent stability against oxidation, especially a saturation magnetization decrement percentage of not more than 17%, and a process for producing the spindle-shaped fine magnetic iron-based alloy particles containing cobalt and iron as the main ingredient, spindle-shaped goethite fine particles containing cobalt which have an average major axis diameter of 0.05 to 0.20 .mu.m, an aspect ratio (major axis diameter/minor axis diameter, hereinunder referred to simply as "aspect ratio") of 4 to 15, an X-ray crystallite size ratio (D.sub.020 /D.sub.110) of 1.5 to 3.5, a uniform particle size, especially the size distribution (standard deviation/major axis diameter) of not more than 0.30 and no inclusion of dendrites, and a process for producing the spindle-shaped goethite fine particles containing cobalt.
The spindle-shaped magnetic iron-based alloy particles containing cobalt and iron as the main ingredients according to the present invention are fine particles which have a uniform particle size, no inclusion of dendrites, a high coercive force, an excellent coercive force distribution, a large saturation magnetization, and in addition, an excellent stability against oxidation. In addition, the spindle-shaped magnetic iron-based alloy particles containing cobalt and iron as the main ingredients according to the present invention are excellent in dispersibility and packing property as a material for a magnetic recording medium. The spindle-shaped goethite particles containing cobalt according to the present invention are suitable for use as a starting material for magnetic particles, especially as a starting material which produces the spindle-shaped fine magnetic iron-based alloy particles containing cobalt and iron as the main ingredients.
Miniaturized and lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording have recently shown a remarkable progress. Especially, video tape recorders (VTR) have rapidly spread wide and the development of miniaturized and lighter-weight VTR's for longer-time recording have been rapid. With this development, magnetic recording media such as a magnetic tape have been strongly required to have a higher performance and a higher recording density.
In other words, magnetic recording media are required to have a higher picture quality and higher output characteristics, in particular, to improve the frequency characteristics and lower the noise level. For this purpose, it is necessary to improve the residual flux density (Br), the coercive force, the dispersibility, the packing property, the surface smoothness of the magnetic recording media such as a magnetic tape and the S/N ratio.
These properties of magnetic recording media have close relation to the magnetic particles used for the magnetic recording media. In recent years, magnetic iron-based alloy particles have attracted attention due to their coercive force and saturation magnetization which are superior to those of conventional iron oxide magnetic particles, and have been put to practical use as magnetic recording media such as digital audio tapes (DAT), 8-mm video tapes, Hi-8 tapes and video floppies. Such magnetic iron-based alloy particles, however, are also strongly demanded to improve the properties.
The relationship between various properties of magnetic recording media and magnetic particles used therefor is described in the following.
In order to obtain a high picture quality, magnetic recording media for VTR's are required to improve (i) the video S/N ratio, (ii) the chroma S/N ratio and (iii) the video frequency characteristics, as is obvious from the description in NIKKEI ELECTRONICS, No. 133, pp. 82 to 105 (1976).
In order to improve the video S/N ratio and the chroma S/N ratio, it is important to improve the dispersibility of the magnetic particles in a vehicle, the orientation property and the packing property of the magnetic particles in a coating film and the surface smoothness of the magnetic recording media. As such magnetic particles, fine particles which have a uniform particle size, no inclusion of dendrites and an appropriate aspect ratio are demanded.
In order to improve the video S/N ratio, it is important to lower the level of the noise caused by a magnetic recording medium. As to the noise of magnetic iron-based alloy particles, it is known that it has a relationship with the X-ray crystallite size of the magnetic iron-based alloy particles.
This phenomenon is shown in, for example, in FIG. 38 on page 123 of the COLLECTED DATA ON MAGNETIC RECORDING MEDIA, Aug. 15 (1985), published by Synthetic Electronics Research. FIG. 38 shows the relationship between the X-ray crystallite size (D.sub.110) of the magnetic iron-based alloy particles and the noise level of the magnetic tape produced therefrom. It is observed from FIG. 38 that the more the X-ray crystallite size lessens, the more the noise level of the magnetic tape lowers.
It is, therefore, effective for lowering the level of the noise caused by a magnetic recording medium to reduce the X-ray crystallite size of the magnetic iron-based alloy particles as much as possible. As described above, in order to improve the video S/N ratio and lower the noise level, magnetic particles are required to have as small an X-ray crystallite size as possible.
In order to improve the video frequency characteristics, it is necessary that the magnetic recording medium has high coercive force (Hc) and residual flux density (Br). In order to enhance the coercive force (Hc) of the magnetic recording medium, magnetic particles having a higher coercive force (Hc) are required.
Since the coercive force of magnetic particles are generally dependent upon the shape anisotropy, a high coercive force is obtained by making the particles as fine as possible or increasing the aspect ratio. This is described in, for example, Japanese Patent Publication (KOKOKU) No. 1-18961 (1989), " . . . The coercive force becomes larger with an increase in the aspect ratio. On the other hand, the coercive force is influenced by the particle size, and in particles having a particle size more than the particle size which produces the superparamagnetism, the coercive force increases in proportion to a reduction in the particle size. It is, therefore, possible to obtain an intended coercive force by appropriately selecting the particle size and the aspect ratio. . . . "
Hi-vision VTRs for home use have recently been developed, and finer particles are demanded as the magnetic iron-based alloy particles which are used for W-VHS tapes of Hi-vision VTRs.
This fact is described in, for example, on pp. 14 to 15 of NIKKEI MATERIAL & TECHNOLOGY No. 128 (1993), "A VTR tape up to the W-VHS standard of a home Hi-vision VTR system has been developed. . . . The VTR tape has a two-layer structure comprising an upper layer composed of a metal (iron) magnetic material and having 0.2 to 0.4 .mu.m in thickness and a lower layer composed of a nonmagnetic material and having 2 to 3 .mu.m in thickness. . . . In a Hi-vision VTR for recording and reproducing a high-definition picture, it is necessary for the purpose of high-density recording to record and reproduce a recording signal at a low noise level. . . . The magnetic iron-based alloy particles in the magnetic layer have also been improved. The particles have been made finer so as to improve the coercive force and the saturation magnetic flux density. . . . Acicular magnetic particles are used but . . . the major axis diameter thereof is 0.1 .mu.m, which is about half of the major axis diameter of the magnetic material for S-VHS. . . . "
It is also required to an excellent switching field distribution (S.F.D.).
This fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 63-26821 (1988), "FIG. 1 is a graph showing the relationship between the S.F.D of the above-described magnetic disk and the recording and reproducing output. . . . As is clear from FIG. 1, the relationship between the S.F.D. and the recording and reproducing output is linear. It indicates that the recording and reproducing output is enhanced by using ferromagnetic powder having a small S.F.D. That is, in order to obtain an output larger than the ordinary output, an S.F.D of not more than 0.6 is required."
There is no end to the demand on these magnetic iron-based alloy particles, and they are required to have a large saturation magnetization in addition to a very small particle size and a high coercive force. This fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 5-98321 (1993), " . . . In order to obtain a high-density magnetic recording medium, it is necessary that the magnetic iron-based alloy particles used are fine particles which have a high coercive force, a large saturation magnetic flux density, an excellent dispersibility and an excellent stability against oxidation. . . . The saturation magnetic flux density changes with the composition of the magnetic iron-based alloy particles, the particle size and the thickness of the oxide film. With respect to the composition, when the magnetic particles are an iron-based alloy particles, addition of cobalt is effective. As to the particle size, the larger the particles, . . . the larger the saturation magnetic flux density naturally becomes. . . . However, since the aspect ratio largely influences the coercive force, it is impossible to make it extremely small. . . . "
Accordingly, a magnetic material for a high-density magnetic recording medium is required to be fine particles which have a uniform particle size, no inclusion of dendrites, a high coercive force, an excellent coercive force distribution, a large saturation magnetization and an excellent stability against oxidation.
Generally, magnetic iron-based alloy particles are obtained by heat-treating in a reducing gas, goethite particles as the starting material, hematite particles obtained by heating and dehydrating the goethite particles, or goethite particles or hematite particles containing metals other than iron, after the heat-treatment, if necessary.
As a method of producing goethite particles as the starting material, there are known (i) a method of producing acicular goethite particles by oxidizing a suspension containing ferrous hydroxide colloid obtained by adding not less than an equivalent of an alkali hydroxide solution to an aqueous ferrous salt solution, introducing an oxygen-containing gas-thereinto at a pH of not less than 11 and at a temperature of not higher than 80.degree. C. (described in, for example, Japanese Patent Publication (KOKOKU) No. 39-5610 (1964), these particles obtained are referred hereinunder as "iron hydroxide-based goethite particles"), (ii) a method of producing spindle-shaped goethite particles by oxidizing a suspension containing FeCO.sub.3 obtained by reacting an aqueous ferrous salt solution with an aqueous alkali carbonate solution, introducing an oxygen-containing gas thereinto (described in, for example, Japanese Patent Application Laid-Open (KOKAI) No. 50-80999 (1975), these particles obtained are referred hereinunder as "iron carbonate-based goethite particles"), and (iii) a method of producing spindle-shaped goethite particles by oxidizing a suspension containing an Fe-containing precipitate obtained by reacting an aqueous ferrous salt solution with a mixed aqueous solution of alkali carbonate and alkali hydroxide, introducing an oxygen-containing gas thereinto (described in, for example, Japanese Patent Application Laid-Open (KOKAI) No. 2-51429 (1990), these particles obtained are referred hereinunder as "mixed alkalis-based goethite particles").
Prior arts for producing these iron hydroxide-based goethite particles, iron carbonate-based goethite particles, mixed alkalis-based goethite particles are disclosed in, for example, Japanese Patent Publication (KOKOKU) Nos. 55-29577 (1970), 1-18961 (1989), 2-57122 (1990) and 3-43323 (1991), Japanese Patent Application Laid-Open (KOKAI) Nos. 63-222404 (1988), 3-49026 (1991), 4-56709 (1992), 4-63210 (1992), 5-62166 (1993), 5-98321 (1993), 6-25702 (1994), 6-36265 (1994), 6-139553 (1994), 6-140222 (1994) and 6-215360 (1994), and EP 0 466 338 A1.
The iron hydroxide-based goethite particles have an aspect ratio as large as 10 but contains dendrites and cannot be uniform particle size. This fact is described in, for example, Japanese Patent Application Laid-Open (KOKAI) No. 5-62166 (1993), " . . . are produced by air-oxidizing the hydrolyzate of a ferrous salt by an alkali hydroxide. According to this method, in order to make the goethite particles finer, for example, a water-soluble silicate -is added to a reaction system. . . . There is a tendency of producing dendrites in particles . . . , the particle size distribution of the magnetic iron-based alloy particles becomes wide. . . . If it is intended that the goethite particles are made extremely fine and that the particle size distribution is made uniform, the hydroxide alkali method has its limitation." When from the iron hydroxide-based goethite particles are produced magnetic iron-based alloy particles, although it is easy to obtain a high coercive force, especially not less than 1720 Oe, it is difficult to reduce the particle size and to obtain a uniform particle size, which makes it difficult to obtain an excellent switching field distribution.
The iron carbonate-based goethite particles are spindle-shaped particles which have a uniform particle size and which do not contain any dendrites, but the aspect ratio is at most about 7 and it is difficult to produce spindle-shaped goethite particles having a large aspect ratio. This fact is described in, for example, Japanese Patent Application Laid-Open (KOKAI) No. 5-62166 (1993), " . . . The spindle-shaped goethite particles obtained by the air-oxidization after hydrolyzing a ferrous salt by an alkali carbonate, are fine particles and have a uniform particle size. . . . It is considered that these goethite particles generally have a small aspect ratio and it is difficult to enhance the coercive force due to the shape anisotropy. . . . " When from the iron carbonate-based goethite particles are produced magnetic iron-based alloy particles, although the produced magnetic iron-based alloy particles show a excellent distribution, it is difficult to have a high coercive force, especially not less than 1720 Oe.
The mixed alkalis-based goethite particles are spindle-shaped particles having a uniform particle size and a large aspect ratio. This fact is described in, for example, Japanese Patent Application Laid-Open (KOKAI) No. 2-51429 (1990), " . . . It is possible to obtain spindle-shaped goethite particle having a large aspect ratio (major axis diameter/minor axis diameter). . . . " The mixed alkalis-based goethite particles have a tendency of producing magnetic fine iron-based alloy particles having a good size distribution and a higher coercive force than the magnetic iron-based alloy particles produced from the iron carbonate-based goethite particles.
The present inventors investigated in detail the magnetic iron-based alloy particles produced from the iron hydroxide-based goethite particles as the starting material in comparison with the magnetic iron-based alloy particles produced from the iron carbonate-based goethite particles as the starting material in accordance with the prior art.
The method disclosed in Japanese Patent Publication (KOKOKU) No. 55-29577 (1970) is a method of producing acicular Fe-Co alloy particles by producing acicular goethite particles uniformly containing Co (which is equivalent to the iron hydroxide-based goethite particles in this specification) and reducing the goethite particles with hydrogen. In Examples 3 and 4 of Japanese Patent Publication (KOKOKU) No. 55-29577 (1970), when 20 mol % and 25 mol % of Co is respectively added, fine particles having a major axis diameter of 0.2 .mu.m and 0.1 .mu.m, respectively, are obtained. However, the coercive force thereof is at most as low as 1120 Oe and 1000 Oe, respectively. In addition, even if the amount of Co added is increased, the coercive force lowers.
The method disclosed in Japanese Patent Publication (KOKOKU) No. 1-18961 (1989) is a method of producing magnetic iron-based alloy particles having a major axis diameter of 0.05 to 0.2 .mu.m, an aspect ratio of 4 to 8 and a coercive force Hc of not less than 1300 Oe from spindle-shaped goethite particles having a major axis diameter of 0.05 to 0.3 .mu.m, a minor axis diameter of 0.015 to 0.04 .mu.m, and an aspect ratio of 3 to 15 (which is equivalent to the iron carbonate-based goethite particles in this specification). All the magnetic iron-based alloy particles obtained in Examples of Japanese Patent Publication (KOKOKU) No. 1-18961 (1989) have a coercive force of not more than 1700 Oe.
The method disclosed in Japanese Patent Publication (KOKOKU) No. 2-57122 (1990) is a method of producing iron magnetic particles or magnetic iron-based alloy particles by depositing a water-soluble salt of Al, Cr, Ce or Nd and an water-soluble borate compound or perborate compound on the surface of an oxide or hydroxide oxide of ferric oxide hydroxide of hematite containing another element such as Ni and Co, and heat-treating the particles in a reducing gas. In Examples 1 to 3 of Japanese Patent Publication (KOKOKU) No. 2-57122 (1990), a maximum coercive force (Hc) of the magnetic iron-based alloy particles is 1840 Oe, and all of these particles are produced from iron hydroxide-based goethite particles. The magnetic iron-based alloy particles in Examples 4 and 5 of Japanese Patent Publication (KOKOKU) No. 2-57122 (1990) are produced from iron carbonate-based goethite particles and have a coercive force (Hc) of 1430 to 1600 Oe.
The method disclosed in Japanese Patent Publication (KOKOKU) No. 3-43323 (1991) is a method of producing magnetic iron-based alloy particles by mixing an aqueous solution containing sodium carbonate and at least one compound selected from the group consisting of water-soluble Co compounds, Ni compounds and Cu compounds with an aqueous solution of a ferrous salt, blowing air into the obtained mixture to produce .alpha.-FeOOH (which is equivalent to the iron carbonate-based goethite particles in this specification), and thereafter reducing the resultant mixture. The coercive force (Hc) of the magnetic iron-based alloy particles obtained is 700 to 1200 Oe.
In the invention described in Japanese Patent Application Laid-Open (KOKAI) No. 63-222404 (1988), although the magnetic iron-based alloy particles having a coercive force (Hc) of not less than 1900 Oe are obtained, since the magnetic iron-based alloy particles is acicular, it is apparent that the iron hydroxide-based goethite particles is used as a raw material and as seen from the Comparative Examples described later, the size distribution of the obtained magnetic iron-based alloy particles is extremely inferior.
The magnetic iron-based alloy particles produced from mixed alkalis-based goethite particles in accordance with the prior art were also investigated.
According to the method described in Japanese Patent Application Laid-Open (KOKAI) No. 4-63210 (1992), when a zinc compound is existent in a suspension containing FeCO.sub.3 or an Fe-containing precipitate obtained by reacting an aqueous ferrous salt solution with a mixed aqueous solution of an alkali carbonate and an alkali hydroxide, it is possible to obtain goethite particles (which is equivalent to the iron carbonate-based goethite particles in this specification) having an improved axis ratio, especially having an aspect ratio of more than 15, and it is possible to increase the coercive force of the magnetic iron-based alloy particles produced. In Examples 11 to 17 of Japanese Patent Application Laid-Open (KOKAI) No. 4-63210 (1992), the magnetic iron-based alloy particles having a coercive force (Hc) of more than 1700 Oe are obtained, but the axis diameter thereof is as large as 0.23 to 0.33 .mu.m.
Japanese Patent Application Laid-Open (KOKAI) No. 5-62166 (1993) discloses spindle-shaped magnetic iron-based alloy particles produced from a starting material of spindle-shaped added metal-containing goethite particles (which is equivalent to the iron carbonate-based goethite particles in this specification) which are produced by air-oxidizing a precipitate slurry obtained by hydrolyzing a ferrous sulfate and a metal salt selected from the group consisting of Ni, Co, Zn and Mn with an alkali carbonate. In the Examples of Japanese Patent Application Laid-Open (KOKAI) No. 5-62166 (1993), the added metal is only Ni, and although the major axis diameter of the magnetic iron-based alloy particles obtained is as small as 0.10 to 0.20 .mu.m, the coercive force thereof is not more than 1700 Oe.
Japanese Patent Application Laid-Open (KOKAI) No. 5-98321 (1993) discloses magnetic iron-based alloy particles produced by growing the crystals of iron hydroxide oxide (iron oxyhydroxide) on the surfaces of particles of the seed crystals mainly containing iron hydroxide oxide which is obtained by blowing an oxidizing gas into a suspension of a mixture of a ferrous salt and an alkali, by adding an aqueous solution of a rare earth compound and/or a silicon compound to the suspension in a nonoxidizing atmosphere in process of the oxidation, aging the suspension at a temperature higher than the oxidation temperature and blowing an oxidizing gas again into the suspension; coating the crystals with a shape-retaining agent, heat-treating the obtained particles and heat-treating the treated particles in a reducing gas. All of the goethite particles described in Examples 1 to 8 of Japanese Patent Application Laid-Open (KOKAI) No. 5-98321 (1993) are iron hydroxide-based goethite particles. The goethite particles described in Example 9 of Japanese Patent Application Laid-Open (KOKAI) No. 5-98321 (1993) are iron carbonate-based goethite particles, but in this case, compounds of a ferrous salt, Nd and Si are not added in the oxidation process to grow crystals, but the surfaces of spindle-shaped goethite particles are coated with a ferrous salt, Nd and Si. The coercive force (Hc) of the magnetic iron-based alloy particles obtained is 1602 Oe.
Japanese Patent Application Laid-Open (KOKAI) No. 6-140222 (1994) discloses magnetic iron-based alloy particles containing a rare earth element, an alkaline earth metal and Al and/or Si at around surface of each particle, and having an average major axis diameter of 0.05 to 0.3 .mu.m, a coercive force of 1600 to 2000 Oe, a saturation magnetization of 100 to 150 emu/g, a specific surface area of 45 to 70 m.sup.2 /g and a X-ray crystallite size as determined by X-ray diffraction of 120 to 170 .ANG..
Japanese Patent Application Laid-Open (KOKAI) No. 6-25702 (1994) discloses magnetic iron-based alloy particles and further containing Ni, Co, one or more rare earth element and Al and/or Si compound, and having a major axis diameter of 0.05 to 0.3 .mu.m, a crystallite size of 130 to 170 .ANG., a specific surface area of 40 to 70 m.sup.2 /g, a coercive force of 1600 to 2000 Oe and a saturation magnetization of not less than 100 emu/g after allowing to stand at 60.degree. C. and 90% RH for 7 days.
In Example 9 of Japanese Patent Application Laid-Open (KOKAI) No. 6-25702 (1994), the stability against oxidation of the magnetic iron-based alloy particles is inferior and further the squareness (.sigma..sub.r /.sigma..sub.s) thereof is not satisfactory. In Example 6 of Japanese Patent Application Laid-Open (KOKAI) No. 6-140222 (1994), the coercive force (Hc) of the magnetic iron-based alloy particles is low, and in Example 7 thereof, although the saturation magnetization of the magnetic iron-based alloy particles is appropriate, the stability against oxidation thereof is inferior and further the squareness (.sigma..sub.r /.sigma..sub.s) thereof is not satisfactory.
From the result of the above-described investigation, it can be said that magnetic iron-based alloy particles produced from iron hydroxide-based goethite particles as a starting material have a coercive force (Hc) of more than 1700 Oe, while it is impossible to obtain fine magnetic iron-based alloy particles which have a coercive force (Hc) of more than 1700 Oe when iron carbonate-based goethite particles are used as a starting material.
That is, no fine magnetic iron-based alloy particles which have balanced properties such as a uniform particle size, no inclusion of dendrites, an appropriate aspect ratio, a high coercive force, an excellent coercive force distribution, a large saturation magnetization and an excellent and balanced stability against oxidation, are produced from iron carbonate-based goethite particles or mixed alkalis-based goethite particles as a starting material.
Accordingly, the technical problem of the present invention is to provide spindle-shaped fine magnetic iron-based alloy particles containing cobalt and iron as main ingredients which have a uniform particle size, especially size distribution (standard deviation/major axis diameter) of not more than 0.25, no inclusion of dendrites, a high coercive force, especially a coercive force (Hc) of 1720 to 2500 Oe and an excellent stability against oxidation, especially a saturation magnetization decrement percentage of not more than 17%.
Further, the technical problem of the present invention is to provide spindle-shaped magnetic iron-based alloy particles containing cobalt and iron as main ingredients which have a uniform particle size, especially a size distribution (standard deviation/major axis diameter) of not more than 0.25, no inclusion of dendrites, an average major axis diameter of 0.05 to 0.12 .mu.m, a coercive force (Hc) of 1850 to 2500 Oe, a saturation magnetization of not less than 130 emu/g and an excellent stability against oxidation, especially a saturation magnetization decrement percentage of not more than 17%, by using mixed alkalis-based goethite particles as a starting material.
Also, the technical problem of the present invention is to provide spindle-shaped magnetic iron-based alloy particles containing cobalt and iron as main ingredients which have a uniform particle size, especially a size distribution (standard deviation/major axis diameter) of not more than 0.25, no inclusion of dendrites, an average major axis diameter of 0.05 to 0.18 .mu.m, a coercive force (Hc) of 1720 to 2200 Oe, a saturation magnetization of not less than 110 emu/g and an excellent stability against oxidation, especially a saturation magnetization decrement percentage of not more than 14%, by using mixed alkalis-based goethite particles as a starting material.
As a result of studies undertaken by the present inventors so as to achieve the technical problem, it has been found that spindle-shaped magnetic iron-based alloy particles containing cobalt and iron as the main ingredients which are produced by so controlling that an average major axis diameter is 0.05 to 0.18 .mu.m, an average minor axis diameter is 0.010 to 0.020 .mu.m, an aspect ratio (major axis diameter/minor axis diameter) is 4 to 15 and X-ray crystallite size D.sub.110 is 120 to 180 .ANG., a size distribution (standard deviation/major axis diameter) of not more than 0.25 and the Co content in the spindle-shaped magnetic iron-based alloy particles is 1.0 to 50.0 atm % of Co (calculated as Co) based on the total Fe, have a high coercive force, especially a coercive force of 1720 to 2500, an excellent coercive force distribution, a saturation magnetization of not less than 110 emu/g and an excellent stability against oxidation, especially a saturation magnetization decrement percentage of not more than 17%. The present invention has been achieved on the basis of this finding.